1. Field of the Invention
The present invention relates to contact type shape measuring apparatuses, and more particularly, to a contact type shape measuring apparatus that precisely measures a surface shape of a diffraction optical element or a molding die used to form the diffraction optical element.
2. Description of the Related Art
With increase in performance of various optical apparatuses such as an image pickup camera, a laser beam printer, a copying machine, and a semiconductor exposure apparatus, requirements for optical elements to be mounted in these optical apparatuses are becoming increasing high. In particular, diffraction optical elements utilizing light diffraction have recently been used in various products. In diffraction optical elements, a phase difference of light is frequently formed by regularly arranging stepped portions having heights ranging from several tens of micrometers to submicrometers so as to cause diffraction. To measure a surface shape of an optical element having a stepped portion or a surface shape of an optical-element molding die, a contact type shape measuring apparatus has been used widely.
Japanese Patent Laid-Open No. 10-19504 discloses an example of a contact type shape measuring apparatus that measures a surface shape of a target surface of an optical element or an optical-element molding die serving as a target object to be measured. In this apparatus, one end of a probe elastically supported by a moving member, such as an XYZ slider, is brought into contact with the target surface and a three-dimensional position of the other end of the probe from a reference point is measured with a laser length measuring device. A holder (housing) including a spring mechanism for holding the probe is provided between the probe and the moving member. By pressing the moving member toward the probe from a balanced position, a contact force can be applied from the probe to the target surface. When the moving member is driven in the XY directions, the probe traces the shape of the target surface in the Z-direction while moving in the XY-directions. The moving member is controlled so as to follow in the Z-direction to maintain a pressing force corresponding to a predetermined contact force. The probe is scanned over the entire target surface to obtain measurement data. Measurement data obtained in a series of scanning operations reflects the shape of the target surface. By analyzing the measurement data, the shape of the target surface can be measured.
In order to cause the probe to perform precise copy scanning of the target surface, the contact force of the probe with the target surface is minimized.
However, in a case in which the contact force is reduced, if a dust, a projection, or a flaw exists on the target surface, the probe is prone to bounce, and the behavior of the probe becomes unstable. Accurate data cannot be obtained during an unstable period from when the probe bounces from the target surface to when vibration is settled.
Japanese Patent Laid-Open No. 2007-57308 discloses a probe that shortens such an unstable period. A force generating unit is provided in a housing of the probe. When a bounce of the probe is detected, for example, on the basis of the relative displacement between the probe and a moving member, acting force is quickly applied to the probe so as to return the probe onto a target surface to be measured. When a controller detects the bounce of the probe, it applies, to the probe, an acting force for cancelling the bounce of the probe. For example, the acting force is applied by multiplying a proportionality coefficient in accordance with the relative displacement or relative speed between the probe and the moving member.
When the target surface has discontinuous stepped portions like the diffraction optical element, there are the following problems.
When the probe passes over an obstacle serving as a disturbance factor, such as dust, a projection, or a flaw, adhering to a continuously extending target surface, the probe is bounced by collision with the disturbance factor. As a result, the probe moves away from the target surface. In contrast, when a surface having a stepped portion like a diffraction optical element is measured, the probe sometimes measures a higher portion of the surface, passes over the stepped portion, and subsequently measures a lower portion of the surface. In this case, after passing over, the probe separates from the surface even if it does not bounce.
FIG. 6 illustrates how to scan a surface having a stepped portion in the related art disclosed in Japanese Patent Laid-Open No. 2007-57308.
A target surface 2a of a target object 2 to be measured has a stepped portion 2s. A probe 1 provided in a moving member (not illustrated) is subjected to copy scanning in a direction D that is orthogonal to a ridge line of the stepped portion 2s and extends beyond the stepped portion 2s. At a tip of the probe 1, a ball formed of a highly abrasive resistant material is provided to contact with the target surface 2a. In FIG. 6, t1b represents the behavior of the center of the ball of the probe 1, and T9B represents the behavior of the unillustrated moving member. For easy viewing, T9b is shown near T1b. The probe 1 leaps over the stepped portion 2s, temporarily separates from the target surface 2a, but falls with time. After that, the probe 1 collides with the target surface 2a again, and is bounced by reactive force resulting from the collision. In general, the moving member that elastically supports the probe 1 is heavier and moves more slowly than the probe 1. For this reason, the moving member is delayed and follows the probe 1 during a period B1. During a period B2, the moving member overshoots beyond the height of the stepped portion 2s. Although the moving member sometimes repeats vibration, it is assumed here, for easy explanation, that the moving member is stabilized after the period B2. If the probe 1 is moved in conformity with the behavior of the moving member during the period B1, acting force is applied to the probe so that the probe does not separate from the moving member. Hence, the probe 1 floats above the target surface 2a. Conversely, during the period B2, the probe 1 is pressed against the target surface 2a with an excessive acting force. If the probe 1 is pressed with an excessive acting force, the contact force inevitably increases, and the probe 1 is bounced more by reactive force received from the target surface 2a. 
In the above-described related art of Japanese Patent Laid-Open No. 2007-57308 in which the probe is controlled on the basis of the relative amount, such as relative displacement/relative speed, between the moving member and the probe, an excessive contact force is prone to occur. This makes it difficult to shorten the unstable period in which the probe is bouncing.